$\forall$$T$:Type, $B$:($T$$\rightarrow$(Top+Top)), $L$:$T$ List.
\\[0ex]concat(map($\lambda$$x$.Case $B$($x$) of inl($a$) $\Rightarrow$ [$a$] ; inr($a$) $\Rightarrow$ nil;$L$))
\\[0ex]$\sim$
\\[0ex]mapfilter($\lambda$$x$.outl($B$($x$));$\lambda$$x$.isl($B$($x$));$L$)